The present invention relates to a device and a method for measuring a moisture value of dielectric materials using at least one microwave resonator. The present invention is also suitable, in particular, for granulation, agglomeration, instantisation, coating and drying processes in a fluidised bed or a moving bulk material.
Generally, the use of microwave technology is known for determining mass and/or moisture. It is disclosed in DE 40 04 119 C2 how the effect of variations in density/mass on a moisture signal is prevented when measuring using microwave resonance. To this end, the alteration in the attenuation of the signal, as is indicated, for example, by the alteration in the resonance width or the resonance amplitude, is normalised to the shift in the resonant frequency. Thus the attenuation signal is normalised to a density state which always produces a resonant frequency shift which is proportional to the density. This two-parametric approach, in which in addition to a shift in the resonant frequency the occurring attenuation is also measured, permits a precise measurement of the moisture independently of the density. In particular, when used in fluidised bed dryers, the two-parametric measuring method makes it possible to distinguish accurately between the different effects on the measuring results. In particular, the effects of the variable degrees of granulation is clearly distinguished from the effects of granulation.
EP 0 970 369 discloses a moisture measurement in the high frequency range on a fluidised bed or a moving bulk material with the attenuation of a resonance mode for determining the moisture. The particle size distribution is thus not directly measured in this case, but it is assumed that the total moisture of the product is a direct measurement of the particle size distribution and thus when the moisture is known, the particle size distribution is also known. As the measured attenuation of the microwave signal, on the one hand, always depends on the quantity of material in the measuring field and, on the other hand, depends on the moisture content thereof, by measuring only the attenuation, the effect of a variation in mass and/or density of the sample material on the moisture signal is not able to be compensated. The method, therefore, may only be used for fluidised bed processes and with moving bulk material, in which such a compensation of the density is not required.
An optical measuring head for the on-line examination of the moisture and/or the particle size of agglomerated particles in granulators or dryers is disclosed in DE 196 45 923 A1.
A device for the continuous measurement of moisture in fluidised beds is disclosed in DE 37 39 538 A1. In this connection, a capacitive measurement takes place in two concentric cylinders, the cylinder axes thereof being arranged approximately parallel to the flow of solids. The moisture content is determined via an alteration to the capacity. As, during the capacitive measuring method, the measuring signals depend on the content of mineral material in the sample material, a distinction between the moisture and density (and/or mass/particle size) is not possible in this method.
A method for monitoring and/or controlling during drying, granulation, instantisation, dragee-making and film-coating processes is disclosed in DE 32 41 544 A1. In this case, the moisture of the exhaust air as well as the moisture of the inlet air is measured and the resulting moisture difference is used for controlling the operating process.
The object of the invention is to provide a method and a device for measuring a moisture value which delivers very accurate results by simple means and, in particular when used for moving bulk material or in fluidised beds, may compensate for variations in the mass and/or density of the sample material.